Animals can also make elective use of a hydrostatic skeleton, its main advantage being that it can be created for as long as it is needed. When it is no longer required, the pressure in the system can be reduced so that nothing remains of the skeleton. True enough, the hydrostatic skeleton is not as reliable as a bone one, and, where the supports have to be permanent, the hydrostatic skeleton has given way to more rigid constructions. But for non-permanent skeletons hydraulics has proved more advantageous. Nature has applied this invention throughout the history of the evolution of the animal kingdom, right from the lowest creatures up to the most developed ones, man included. Cavernous bodies employing blood as the working fluid is a good example.
Hydrodynamic mechanisms are still more fascinating. These range from extremely primitive devices to rather sophisticated ones. The most primitive of them include the excretory siphon-tube in bivalve mollusks. These living creatures derive oxygen and food, microscopic organisms and particles of plant and animal matter from the surrounding water, which they suck into the mantle cavity. The water saturated with carbon dioxide and polluted with excretory products is ejected through a special siphon-tube. The mollusk, no doubt, wishes the waste material to be removed far away from its body so that it would not return into its mantle cavity. This explains why the excretory siphon-tube is rather long, though it has no special muscle and cannot stretch. When the shell is closed and water stops moving into the mantle cavity, the siphon-tube contracts, but as soon as the fluid resumes its flow, the siphon straightens and stretches.
The hydrodynamic (water-vascular) mechanisms in the spider’s legs are concerned with locomotion. These eight-legged creatures whose legs consist of six or seven segments flex them, like all animals do, by contracting certain specific muscles, but extend them by increasing pressure within the chitin-clad legs.